This invention relates in general to internal drum scanner assemblies and laser imaging systems incorporating such scanner assemblies. In particular, the present invention relates to a translation system for moving a laser or optical scanning system relative to media being scanned.
Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of medical image scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and an image management subsystem for coordinating the operation of the laser imager and the film processor.
The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the image management subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film.
The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists).
Film exposure systems are used to provide exposure of the image on photosensitive film. Known film exposure systems include a linear translation system and a laser or optical scanning system. The laser scanning system includes a laser scanner with unique optical configurations (i.e., lenses and mirrors) for exposure of the image onto the film. The linear translation system provides for movement of the laser scanning system in a direction perpendicular to the scanning direction, such that a fall image may be scanned on a piece of photosensitive film.
In an internal drum type laser scanner assembly, a piece of film is positioned onto a film platen, wherein the film platen has a partial cylindrical or partial drum shape. The photosensitive film is positioned against the film platen. The laser or optical scanning system is positioned at the center of curvature of the photosensitive film for scanning a scan line across the photosensitive film surface. A linear translation system moves the laser or optical scanning system lengthwise along a longitudinal axis as defined by the center of curvature of the film to expose an entire image onto the film.
Traditional linear translation systems include three main components, a stationary member, a moving member (e.g., a carriage), and a drive mechanism. In a linear translation system where rigidity, positional accuracy, and high load carrying capacity are required, lead screw mechanisms are preferred as the drive mechanism. Belts and cable systems are used in systems characterized by flexibility, light loads, and low costs, such as plotters and ink jet printers.
Known linear translation systems are usually designed for positional repeatability. Although such systems work well for positional repeatability type scanning operations, such linear translation systems were not designed to minimize velocity variation which is critical for imaging continuous tone photosensitive film. In continuous laser scanning applications, velocity variations cause the scan lines to be unevenly spaced and result in a variety of image artifacts on the photosensitive film.
U.S. Pat. No. 6,064,416, issued May 16, 2000, inventors Esch et al., discloses an optics translation module with a single drive cable. The optics translation module uniformly places laser scan lines to form complete images. The translation direction is perpendicular to the scan line direction.
The optics translation module, controls image quality in the cross scan direction. Speed variation will be reflected in images as bands or streaks of non-uniform densities on film. Unstable motion during the translation of the optics module causes inaccurate placement of pixels, resulting in a variety of image artifacts.
In this design, a translation carriage with kinematic support on a pair of cylindrical rails is used. The carriage, with precision mounting surfaces, serves as an interface between the optics module and the optics translation module. Kinematic support is achieved by a pair of V-shaped bearing surfaces and a flat sliding surface built into the carriage. When the carriage slides on the rails during translation, there is little friction on the carriage in the translation direction, while its position is rigidly determined in the other directions.
Weight of the carriage and the attached optics module is necessary for maintaining contact between the carriage and the rails. If the carriage is lifted from the rails, the kinematic support will not function properly. The position of the carriage will then be undetermined.
The optics module is attached to the translation carriage with position reference at an edge formed by two perpendicular planes. This ensures easy mounting of the optics module to the translation module.
The support points of the carriage need to enclose the center of gravity of the optics module and carriage assembly.
A cable drive mechanism is used for driving the translation carriage. For a carriage that relies on kinematic mounting, the drive mechanism should exert as little force as possible in the directions perpendicular to the translation direction. The cable drive mechanism satisfies this requirement. Cable drive is also suitable in this application because of the low load and low mass nature of the carriage and optics module.
The cable is driven by a pair of pulleys, one of which is the drive pulley and the other an idler. The pulleys have 90-degree V-grooves for holding the cable at its desired location.
The drive pulley for the cable is attached to the coaxial with a circular flywheel. The flywheel is driven by a stepping motor through friction drive. On the shaft of the stepping motor, a polyurethane tire is mounted for driving the flywheel through friction.
In order to maintain desired speed uniformity, to better than 0.25% error for motor once-around, the tire needs to be ground on the motor after it is mounted on the motor shaft.
A nylon coated steel cable is used. The cable needs to be strong (i.e., high Young""s modulus) so that the spring constant of the cable in the longitudinal direction is high. For durability of the cable, it needs to be flexible enough to be used with the pulleys.
For long term performance stability of the mechanism, a cable tensioner is necessary. In this design, the cable tensioner is a compression spring.
Mass of the optics module and the carriage, along with the spring constants of the cable and cable tensioner spring, determine the resonant frequency of the translation module. Since it is desirable to increase the resonant frequency, reduced total mass that is attached to the cable is a design consideration.
There is a need for a translation system which overcomes the problems and satisfies the needs discussed before.
According to the present invention, there is provided a translation system which satisfies the needs and overcomes the problems of known systems.
According to a feature of the present invention, there is provided a translation system comprising: a linear support having first and second opposite ends; a carriage slid ably mounted on said support for movement in reciprocal linear directions between said first and second ends of said support; a rotary drive rotatable in opposite rotary directions located at one end of said support; a rotatable member mounted at the other end of said support; and a multiple cable assembly attached to said carriage and extending around said rotary drive and said rotatable member moving said carriage in said reciprocal linear directions as a function of rotation of said rotary drive in said opposite rotary directions.
The invention has the following advantages.
1. Higher rigidity over single drive cable systems.
2. Ease in tracking over metal belt system, ease of alignment, assembly and adjustment over lead screw systems.
3. Overall system cost can be substantially lower that those for a lead screw or a linear motor system with equivalent performance.
4. The spring stiffness of the drive system is increased approximately by a factor equal to the number of cables used. Independently driven and tensioned cables are easy to align, assemble and adjust.
5. Multiple cable driven system significantly reduces system sensitivity to external noise disturbance, when compared with a single cable drive system.